A 4-wire resistive touch screen is an electronic device that registers when a physical touch may occur on the screen. Generally, the structure of a 4-wire resistive touch screen is well known. FIG. 1 illustrates a typical 4-wire resistive touch screen 100. The screen 100 may include a first plastic film layer (a Y layer) 110 with a transparent resistive coating on a bottom side, a pair of Y+ and Y− electrodes 112 at the vertical edges of the first layer 110. The screen 100 may further include a second plastic film layer (an X layer) 120 spaced apart from the first layer 110 with a transparent, resistive coating on a top side and a pair of X+ and X− electrodes 122 at the horizontal edges of the second layer 120. The screen 100 may also include an LCD screen 130 positioned below the two plastic film layers 110, 120 that may display image content viewable through the two plastic film layers 110, 120.
During a touch operation, a user touches a point on the touch screen 100 which causes the first layer 110 to deflect and make contact with the second layer 120. Approximate X-Y Cartesian coordinates of the point of contact are then determined. In a first phase, voltage is driven on the Y layer (say, layer 110) and a voltage is read from a single electrode of the X layer (say, layer 120). In a second phase, a voltage is driven on the X layer, and a voltage is read from a single electrode of the Y layer. A high impedance input device is used to read voltages from the sensing layer in each phase, which minimizes voltage losses in the sensing layer. Thus, voltages sensed at the electrodes of the sensing layer represent the voltage at the point of contact between the two layers. For ease of reference, the layer that is driven by the applied voltage may be called the “active” layer, and the sensing layer may be called the “passive” layer. Voltages read through each operation phase are digitized and converted to a value representing the single-point of contact in the X-Y axis at which the layers touch each other.
It may be desirable for a user to touch a screen in single or dual points of contact during a touch operation. Some touch screen systems have attempted to determine the locations for dual point touches, but these systems involve special screens or are inefficient.
Accordingly, there is a need in the art for techniques to determine the locations of dual touches performed on a four-wire resistive touch screen.